1. Field of the Invention
The invention relates to an image processing method formed by x-ray imaging, and containing a portion corresponding to an x-ray absorption filter. The invention also relates to an image processor to process an image formed by x-ray imaging, which contains a portion corresponding to an x-ray absorption filter. The invention further relates to an x-ray examination apparatus comprising an x-ray absorption filter between an x-ray detector and an x-ray source for irradiating an object to form an x-ray image on the x-ray detector.
2. Description of the Related Art
An image processing method of said kind is known from the U.S. Pat. No. 4,670,896; therein are disclosed an x-ray examination apparatus and the use thereof to form on the x-ray detector the x-ray image containing a portion corresponding to the x-ray absorption filter.
The cited reference teaches that a substantial difference in image-contrast can occur between parts of an x-ray image formed due to irradiation of the object by x-rays. Brightness differences forming image contrast arise e.g. in peripheral angiography because part of the x-ray beam does not penetrate the object, notably a patient to be examined, and is incident directly on the x-ray detector, or for example in cardio-examination because the object exhibits substantial differences in x-ray absorption, viz. a patient's heart having comparatively high x-ray absorption is surrounded by lung-tissue which is relatively transparent for x-rays. In order to make the dynamic range of the x-ray image coincide as well as possible with the contrast in the region of interest, x-ray absorption filters e.g. having the form of x-ray shutters, are arranged between the x-ray source and the object. The x-ray shutters are positioned to intercept a part of the x-ray beam so as to shield relatively transparent parts of the patient's anatomy from the x-rays. The use of such x-ray shutters leads to diminishing the x-ray dose to which a patient is exposed to produce an x-ray image. Moreover, an x-ray dose whereto a person who operates the x-ray examination apparatus is unintentionally exposed is diminished by the use of x-ray image x-ray shutters appear as regions having extremely high absorption. Consequently, in inverted images, as are often used by radiologists for examination, x-ray shutters appear as bright areas which distract attention from medical information. Furthermore, when a plurality of images made by x-ray irradiation is brought together onto a hard copy, such as an x-ray film, a substantial portion of the area of the x-ray film is wasted when said images contain areas corresponding to x-ray shutters. Thus, making images, in accordance with the cited reference, by irradiating an object by an x-ray beam and employing the x-ray absorption filter arranged between the x-ray source and the x-ray detector has several shortcomings and disadvantages.